Conventionally, electrical junction boxes are provided at appropriate positions in electrical systems of automobiles and the like, and thus power is distributed from a battery to various loads with good space efficiency. Specifically, a bus bar circuit unit in which a power source-side bus bar connected to a power source line such as a battery and a ground-side bus bar connected to a ground line are arranged on an insulated board is accommodated in the electrical junction box, and thus power is distributed.
If water infiltrates such an electrical junction box, there is a risk that a short circuit or the like occurs in the bus bar circuit unit. Therefore, certain waterproofing measures are taken for the electrical junction box in consideration of a water splashing that is assumed to occur during use of the vehicle. As mentioned in Japanese Patent No. 4585980 (Patent Document 1), a measure in which gaps of the case of the electrical junction box are sealed with a sealing material or the like, and a measure in which a drainage slope is provided at an appropriate position in the case to facilitate the drainage of water that infiltrated the case are known, for example.
However, a conventional waterproofing structure of the electrical junction box is made for automotive vehicle operation, and therefore, such a waterproofing structure is not effective in an unexpected situation in which water infiltrates the electrical junction box due to tsunami, flood, or the like in a case of a disaster such as the Great East Japan Earthquake. Many instances have been reported in which the electrical junction box located near the battery in the vehicle caught fire after the unexpected infiltration of water, causing vehicle fire, which has been regarded as a problem.
It is necessary to urgently develop some measure for preventing the electrical junction box from catching fire after such unexpected infiltration of water. However, providing the electrical junction box with a waterproofing structure that is made to withstand a case of the infiltration of water due to tsunami, flood, or the like has risks of not only leading an increase in the size and the cost of the electrical junction box but also impairing the functions of the electrical junction box during normal use of the vehicle, and therefore, it is difficult to say that such a measure is practical. Accordingly, there has been the demand for an effective measure capable of preventing the electrical junction box after the infiltration of water from catching fire.
The present invention has been achieved in light of the above-described circumstances, and the problem to be solved of the present invention is to provide an electrical junction box having a novel structure capable of preventing vehicle fire after the infiltration of water without depending on a waterproofing structure of the electrical junction box.
As a result of intensive research by the inventors of the present invention on a cause of fire after the infiltration of water into a vehicle, it was found that particularly in an electrical junction box that is directly connected to a battery, fires broke out in a part where a power source-side bus bar made of copper that was connected to a power source line and a ground-side bus bar made of copper that was connected to a ground line were arranged adjacent to each other. That is, when water containing electrolytes, such as salt water, infiltrates a vehicle, electrolysis occurs between the power source-side bus bar and the ground-side bus bar having a relatively large potential difference therebetween, and thus cuprous oxide (Cu2O), which is an oxide of copper, is precipitated on the anode. When the water recedes, the precipitated cuprous oxide is deposited between the power source-side bus bar and the ground-side bus bar, and thus a short-circuit path made of a deposition of the cuprous oxide is formed between the two bus bars. When current continues to be conducted from the battery even after the infiltration of water, and the temperature rises to some extent, the resistance of the cuprous oxide decreases, and thus a short circuit occurs. It was newly found that the insulated board burned due to the heat at that time, thus causing a fire. The present invention was achieved based on the new findings.